Symmetrical inductively coupled plasma source with coaxial rf feed and coaxial shielding

ABSTRACT

A plasma reactor has an overhead multiple coil inductive plasma source with symmetric RF feeds and symmetrical RF shielding around the symmetric RF feeds.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/666,224 filed Nov. 1, 2012 entitled SYMMETRICAL INDUCTIVELYCOUPLED PLASMA SOURCE WITH SYMMETRICAL FLOW CHAMBER by Andrew Nguyen, etal.; U.S. patent application Ser. No. 13/666,245 filed Nov. 1, 2012entitled INDUCTIVELY COUPLED PLASMA SOURCE WITH PLURAL TOP COILS OVER ACEILING AND AN INDEPENDENT SIDE COIL, by Andrew Nguyen, et al.; and U.S.patent application Ser. No. 13/666,280 filed Nov. 1, 2012 entitledINDUCTIVELY COUPLED PLASMA SOURCE WITH MULTIPLE DIELECTRIC WINDOWS ANDWINDOW-SUPPORTING STRUCTURE by Andrew Nguyen, et al., all applicationswhich claim the benefit of U.S. Provisional Application Ser. No.61/673,937 filed Jul. 20, 2012 entitled SYMMETRICAL MULTIPLE COAXIAL ICPSOURCE AND SYMMETRICAL FLOW CHAMBER, by Andrew Nguyen et al., andassigned to the present assignee. All of the above applications arehereby incorporated by reference in their entirety.

BACKGROUND

1. Field

Embodiments of the present invention are generally concerned with aplasma processing reactor chamber for processing workpieces, in whichplasma is generated by inductive coupling of RF power to process gasesinside the chamber.

2. Description of the Related Art

Electronic devices such as integrated circuits, flat panel displays andthe like, are fabricated by a series of processes, in which thin filmlayers are deposited on substrates and etched into desired patterns. Theprocess steps may include plasma-enhanced reactive ion etching (RIE),plasma-enhanced chemical vapor deposition (CVD), plasma-enhancedphysical vapor deposition (PVD).

Uniform distribution of etch rate or deposition rate across the entiresurface of the substrate is essential for successful fabrication. Suchuniformity is becoming more difficult to achieve, as substrate size isincreasing and device geometry is shrinking. In particular, inductivelycoupled plasma sources can have two concentrically arranged coilantennas over the chamber ceiling, so that uniformity of etch ratedistribution can be optimized by adjusting the different RF power levelsdelivered to the different coil antennas. As workpiece diameter andchamber diameter increase, we have found this approach is not adequate,as the larger size increases the difficulty of attaining the requisiteprocess uniformity. Various sources of process non-uniformity, such aschamber design asymmetries, temperature distribution non-uniformitiesand gas distribution control become more important.

SUMMARY

A plasma reactor comprises a window assembly adjacent a processingregion, inner, middle and outer coil antennas adjacent the windowassembly, and inner, middle and outer current distributors coupled tothe inner, middle and outer coil antennas, respectively. A ceiling plateoverlies the window assembly. First, second and third RF power terminalsare disposed at the ceiling plate. First, second and third axial RFpower feeds are connected between respective ones of the first, secondand third RF power terminals and the inner, middle and outer currentdistributors. The third axial RF power feed comprises a hollow axialouter RF power distribution cylinder surrounding the first and secondaxial RF power feeds.

In one embodiment having coaxial symmetry, the second axial RF powerfeed comprises a hollow axial middle RF power distribution cylindersurrounding the first axial RF power feed. In a related embodiment, thefirst axial RF power feed comprises a center RF connection rod, and thecenter RF connection rod, the hollow middle RF power distributioncylinder and the outer RF distribution cylinder are coaxial. In arelated embodiment, the inner, middle and outer coil antennas arecoaxial with the center RF connection rod.

In one embodiment, the plasma reactor further comprises a plenum platespaced from and below the ceiling plate and comprising a centralopening. In a related embodiment, the outer RF distribution cylinderextends through the central opening with a gap between the plenum plateand the outer RF distribution cylinder, and a radial flange extends fromthe outer RF distribution cylinder and overlies the gap.

In one embodiment, the plasma reactor further comprises: (a) pluralradial middle arms extending outwardly from the middle RF distributioncylinder and plural axial middle legs extending from the plural radialmiddle arms to spaced apart locations on the middle current distributor,and (b) plural radial outer arms extending outwardly from the outer RFdistribution cylinder and plural axial outer legs extending from theplural radial arms to spaced apart locations on the outer currentdistributor.

In a related embodiment, a ground plate is provided below the plenumplate and above the middle current distributor, the plural axial middlelegs extending through the ground plate. In one embodiment, the pluralaxial outer legs extend through the ground plate.

In a related embodiment, each one of the inner, middle and outer coilantennas comprises plural conductors having RF supply ends, and each oneof the inner, middle and outer current distributors comprises an axiallysymmetric hollow body comprising a bottom facing the supply ends of theplural conductors of the corresponding one of the inner, middle andouter coil antennas.

In a further embodiment, the plasma reactor further comprises pluralspaced-apart reactance elements connected between the plenum plate andthe outer RF power distribution cylinder. In one embodiment, thereactance elements comprise discrete capacitors.

In a yet further embodiment, RF shielding is provided, comprising: (a) ahollow cylindrical inner shield between the center RF connection rod andthe middle RF distribution cylinder, and (b) a hollow cylindrical uppershield between the middle RF distribution cylinder and the outer RFdistribution cylinder, the upper shield comprising a bottom edge.

In one embodiment, the plenum plate is spaced from and below the ceilingplate and comprises a central opening, the upper shield extendingthrough the central opening and defining an inner gap between the plenumplate and the upper shield. In a related embodiment, a skirt extendsradially out from the upper shield near the bottom edge and overlies theinner gap.

In a related embodiment, the shielding further comprises: (a) a middlecylindrical shield extending axially from the plenum plate toward theground plate and surrounding the plural axial middle legs, (b) a lowercylindrical shield extending downwardly from the ground plate, and (c) abottom shield extending downwardly from the bottom edge of the lowercylindrical shield. In a related embodiment, a cylindrical base extendsaxially from the bottom shield, the cylindrical base surrounding themiddle coil. In one embodiment, the bottom shield comprises a hollowtruncated cone. In one embodiment, the shielding further comprises afloor encircled by the cylindrical base.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the exemplary embodiments of the presentinvention are attained can be understood in detail, a more particulardescription of the invention, briefly summarized above, may be had byreference to the embodiments thereof which are illustrated in theappended drawings. It is to be appreciated that certain well knownprocesses are not discussed herein in order to not obscure theinvention.

FIG. 1 is a cut-away view of a plasma reactor of an embodiment theinvention.

FIG. 1A is an enlarged view of an upper section of the reactor of FIG.1.

FIG. 1B is an enlarged view of a lower section of the reactor of FIG. 1.

FIG. 2 illustrates an inner zone inductive RF power applicator of thereactor of FIG. 1.

FIG. 3 illustrates an intermediate or middle zone inductive RF powerapplicator of the reactor of FIG. 1.

FIG. 4 illustrates an outer zone inductive RF power applicator of thereactor of FIG. 1.

FIG. 5 illustrates a conductive RF power feeder for the RF powerapplicator of FIG. 3.

FIG. 6 illustrates a conductive RF power feeder for the RF powerapplicator of FIG. 4.

FIG. 7 is a cut-away cross-sectional view of a portion of a lid assemblyof the reactor of FIG. 1.

FIG. 8 is a plan view of a heater layer covering a disk-shapeddielectric window of the lid assembly of FIG. 7.

FIG. 9 is an orthographic projection of a heater layer covering acylindrical dielectric window depicted with the lid assembly of FIG. 7.

FIG. 10 is a plan view of the lid assembly of FIG. 7.

FIG. 11A is a plan view corresponding to FIG. 10 depicting gas flowpassages in a gas flow plate of the lid assembly.

FIG. 11B is a view of an opposite side of the gas flow plate of FIGS. 7and 11A.

FIG. 12 is a plan view corresponding to FIG. 10 and depicting gas flowpaths to a center hub.

FIG. 12A is an orthographic projection corresponding to a portion ofFIG. 12 depicting encasement of a gas flow conduit in a portion of theheater layer of FIG. 8.

FIG. 12B is a cut-away elevational view corresponding to FIG. 12A.

FIG. 13 is an enlarged cut-away view of a center gas disperser of thereactor of FIG. 1.

FIG. 14 is a plan view of the center gas disperser of FIG. 13.

FIG. 15 is a cross-sectional view taken along lines 15-15 of FIG. 14.

FIG. 16 is a cross-sectional view taken along lines 16-16 of FIG. 14.

FIG. 17 is a cross-sectional view taken along lines 17-17 of FIG. 1B.

FIG. 18 is a cross-sectional view taken along lines 18-18 of FIG. 1B.

FIG. 19 is a view corresponding to FIG. 1A and depicting cooling airflow paths.

FIGS. 20A and 20B are block diagrams of alternative embodiments of RFpower sources for the RF power applicators of FIG. 1A.

FIG. 21 is a block diagram of a control system controlling the reactorof FIG. 1.

FIG. 22 is a cross-sectional elevational view of a further embodiment inwhich RF feeds to the coil antennas have triangular symmetry.

FIG. 23 depicts an assembly including an outer coil RF feed of FIG. 22.

FIG. 24 depicts an assembly including a middle coil RF feed and an outercoil RF feed of FIG. 22

FIG. 24A is a cross-sectional view along lines 24A-24A of FIG. 23.

FIG. 25 depicts the outer coil RF feed of FIG. 22.

FIG. 26 depicts the middle coil RF feed of FIG. 22.

FIG. 27 depicts the inner coil RF feed of FIG. 22.

FIG. 28 is a cross-sectional elevational view of a further embodiment inwhich RF feeds to the coil antennas have coaxial symmetry.

FIG. 29 depicts an assembly including a middle coil RF feed and an outercoil RF feed of FIG. 28.

FIG. 30 depicts an assembly including an outer coil RF feed of FIG. 28.

FIG. 31 depicts an assembly including a middle coil RF feed of FIG. 28.

FIG. 32 is a cross-sectional elevational view of a further embodimentwith coaxial RF feeds to the coil antennas having a symmetrical array ofcapacitors connected in parallel with the outer coil RF feed.

FIG. 33 depicts an assembly including an outer coil RF feed of FIG. 32.

FIG. 34 depicts the assembly of FIG. 33 further depicting the array ofcapacitors of FIG. 32.

FIG. 35 is a cross-sectional elevational view of a further embodimenthaving coaxial shielding between the RF feeds.

FIG. 36 is an orthographic projection of the coaxial shielding of FIG.35.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation. It is to be noted, however, that the appendeddrawings illustrate only exemplary embodiments of this invention and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

DETAILED DESCRIPTION

A plasma reactor 10 depicted in FIG. 1 includes an upper portion 20depicted in the enlarged view of FIG. 1A and a lower portion 30 depictedin the enlarged view of FIG. 1B. Referring to FIGS. 1, 1A and 1B, theplasma reactor 10 includes a plasma processing chamber 100 having a sidewall 105 and a lid assembly 110. The side wall 105 has an axiallysymmetrical shape, such as a cylinder. The side wall 105 includes anaxially symmetrical (e.g., cylindrical) dielectric side window 106 and achamber liner 107, which may be formed of metal. A workpiece support 115inside the chamber 100 includes a pedestal 120 having a workpiecesupport surface 121 facing the lid assembly 110 for holding a workpiece122, and a post 125 supporting the pedestal 120. A processing region 101of the chamber 100 is confined by the lid assembly 110, the pedestal 120and the side wall 105. The pedestal 120 may include an insulatedinternal electrode 130. Optionally, an electrostatic chucking (ESC)voltage and/or RF plasma bias power may be supplied to the internalelectrode 130 via a cable 132 extending through the post 125. The cable132 may be coupled to an RF bias power source (such as an RF impedancematching network and/or an RF power generator) as an RF bias feed to theelectrode 130. The cable 132 and may be provided as a coaxialtransmission line, which may be rigid (or flexible), or as a flexiblecoaxial cable.

Plasma source power is inductively coupled into the processing region101 by a set of coil antennas, including an inner coil antenna 140, amiddle coil antenna 150 and an outer or side coil antenna 160, all ofwhich are concentrically disposed with respect to each other and arecoaxial with the axis of symmetry of the side wall 105. The lid assembly110 includes a disk-shaped dielectric window 112 through which the innerand middle coil antennas 140 and 150 inductively couple RF plasma sourcepower into the processing region 101. The disk-shaped dielectric window112 is coaxial with the side wall 105 and has a disk-plane parallel withthe plane of the workpiece support surface 121. The side coil antenna160 inductively couples RF plasma source power into the processingregion 101 through the cylindrical dielectric side window 106. Thedielectric windows 106 and 112 may be referred to collectively as awindow assembly.

Referring to FIGS. 1A and 2, in one embodiment, the inner coil antenna140 includes four wire conductors 140-1 through 140-4, each onehelically wound about a constant radius along an arc length of 180degrees, their ends being staggered (i.e., offset along acircumferential direction) at uniformly spaced 90 degree intervals, asdepicted in FIG. 2. Uniform and symmetrical distribution of RF power tothe wire conductors 140-1 through 140-4 is provided by an RF currentdistributor in the form of an inverted metal bowl 142 having a circularbottom edge 144 contacting the top ends of each of the wire conductors140-1 through 140-4, and a lid 146 connected to an inner RF feed rod148. The bottom ends of the four wire conductors 140-1 through 140-4 aregrounded by connection to an inner ground shield 149 (FIG. 1A) in theform of a cylindrical metal sleeve coaxial with the coil antenna 140 andlying between the inner and middle coil antennas 140 and 150. The innerground shield 149 provides a uniform and symmetrical distribution ofground current from the four wire conductors 140-1 through 140-4, andfurther provides RF shielding or isolation between the inner and middlecoil antennas 140 and 150, by suppressing mutual inductance betweenthem. This enhances independent control of the inner and middle coilantennas 140, 150.

Referring to FIGS. 1A and 3, in one embodiment, the middle coil antenna150 includes four wire conductors 150-1 through 150-4, each onehelically wound about a constant radius along an arc length of 180degrees, their ends being staggered at uniformly spaced 90 degreeintervals, as depicted in FIG. 3. Uniform and symmetrical distributionof RF power to the wire conductors 150-1 through 150-4 is provided by anRF current distributor in the form of a cylindrical metal sleeve 152having a circular bottom edge 154 contacting the top ends of each of thewire conductors 150-1 through 150-4, and a circular top edge 156connected to a circular array of four axial RF feed rods 158. RF poweris fed to the RF feed rods 158 by a conductor structure depicted in FIG.5, which is described later in this specification.

Referring again to FIG. 1A, the bottom ends of the four wire conductors150-1 through 150-4 are grounded by connection to a middle ground shield159. The middle ground shield 159 may be in the form of a cylinder.However, in one embodiment depicted in dashed line in FIG. 1A, the topof the middle ground shield 159 is a metal ring 159-1 coaxial with thecoil antenna 150. Four conductive legs 159 a through 159 d (of whichonly the legs 159 a and 159 c can be seen in the view of FIG. 1A) extendaxially downward from the ring 159-1 and have bottom ends contacting thebottom ends of the four conductors 150-1 through 150-4. The middleground shield 159 provides a uniform and symmetrical of ground currentfrom the four wire conductors 150-1 through 150-4.

Referring to FIGS. 1A and 4, the side coil antenna 160 is disposed belowthe plane of the disk shaped dielectric window 112 and surrounds thecylindrical dielectric side window 106. In one embodiment, the side coilantenna 160 includes eight wire conductors 160-1 through 160-8, each onehelically wound about a constant radius along an arc length of 90degrees, their ends being staggered at uniformly spaced 45 degreeintervals, as depicted in FIG. 4. Uniform and symmetrical distributionof RF power to the wire conductors 160-1 through 160-8 is provided by anRF current distributor in the form of an inverted metal bowl 162 (FIG.1A) having a circular bottom edge 164 attached to respective axialconductors 161-1 through 161-8 (of which only the axial conductors 161-1and 161-5 are visible in the view of FIG. 1A) contacting the top ends ofthe wire conductors 160-1 through 160-8 respectively. The inverted metalbowl 162 further has a circular top edge 166 connected to a circulararray of eight uniformly spaced axial RF feed rods 168. A cylindricalouter chamber wall 170 surrounds the side coil antenna 160 and isgrounded. The bottom ends of the eight wire conductors 160-1 through160-8 are grounded by connection to the outer chamber wall 170. Whilethe described embodiments include direct connection to ground of thecoil antennas 140, 150 and 160 by the ground shields 149 and 159 and theouter chamber wall 170, respectively, the connection to ground may notneed to be a direct connection, and instead the connection to ground maybe through elements such as capacitors, for example.

Referring to FIG. 5, the four axial RF feed rods 158 associated with themiddle coil antenna 150 extend to four radial RF feed rods 172 connectedto a common axial feed rod 174. Referring to FIG. 6, the eight axial RFfeed rods 168 associated with the side coil antenna 160 extend to eightradial RF feed rods 176 connected to a common axial feed rod 178. Theaxial RF feed rod 148, the common axial feed rod 174 and the commonaxial feed rod 178 couple RF power to the respective coil antennas 140,150 and 160. The power may be supplied from a common RF source or fromdifferent RF sources such as RF matches (RF impedance matching networks)180 and 182. As will be described below with reference to FIG. 20B, anRF impedance matching network may be employed having dual outputs inorder to drive two of the coil antennas with a first RF generator, whilea second RF generator and a second RF impedance matching network drivesthe third coil antenna. Alternatively, as will be described below withreference to FIG. 20A, three RF generators may separately drive thethree coil antennas through three respective RF impedance matchingnetworks. In yet another embodiment, a single RF power generator maydrive all three coil antennas through an RF impedance matching networkhaving three outputs. In some implementations of the foregoingembodiments, the RF power levels applied to the different coil antennasmay be separately adjusted in order to control radial distribution ofplasma ion density. While described embodiments include the three coilantennas 140, 150 and 160, other embodiments may include only one or twoof the three described coil antennas 140, 150 and 160.

Only the axial RF feed rod 148 is symmetrically located at the axis ofsymmetry of the side wall 105, while the axial feed rods 174 and 178 arelocated off-center, as depicted in FIGS. 1A, 5 and 6. This feature isasymmetrical. The axial RF feed rods 148, 158 and 168 are arrayedsymmetrically relative to the axis of symmetry of the side wall 105. Agenerally disk-shaped conductive ground plate 184 generally parallelwith the workpiece support plane 121 contains openings through which theaxial RF feed rods 148, 158 and 168 extend. The ground plate 184provides separation between an upper region containing thenon-symmetrically arranged axial feed rods 174 and 178 (and an upperportion of the RF feed rod 148 which is symmetrically located), and alower region containing only symmetrical features such as the axial RFfeed rods 148, 158 and 168. The RF feed rods 148, 158 and 168 areelectrically insulated from the ground plate 184. The ground plate 184electromagnetically shields the processing region 101 from the effectsof the asymmetric features above the ground plate 184 and alsopreventing skew effects in plasma processing of the workpiece 122.

Referring to FIGS. 1 and 7, the disk-shaped dielectric window 112 has adiameter less than the diameter of the outer chamber wall 170. Thewindow 112 is supported at its periphery by an annular top gas plate 200(described later in this specification) that spans the gap between theouter chamber wall 170 and the window 112, while maintaining the spacebelow the window 112 free of structure that would otherwise inhibitinductive coupling of RF power into the processing region 101. Thechamber diameter is therefore not limited by the diameter of thedisk-shaped dielectric window 112. The inner and middle coil antennas140 and 150 (coextensive with the disk-shaped dielectric window 112) maycontrol plasma ion density distribution within an intermediate zone of adiameter smaller than that of the workpiece or wafer 122. Plasma densityin an outer zone is governed by the side coil antenna 160 through thecylindrical dielectric window 106. This affords control of plasma iondensity distribution across the entire wafer without requiring aconcomitant increase in diameter of the disk-shaped dielectric window112.

As referred to above, the annular top gas plate 200 supports thedisk-shaped dielectric window 112 and spans the gap or distance betweenthe outer chamber wall 170 and the periphery of the disk-shapeddielectric window 112. The top gas plate 200 includes an annulus 202surrounding an opening 204. A top inner edge 202 a of the annulus 202underlies and supports an outer edge 112 a of the dielectric window 112and surrounds the opening 204. A bottom outer edge 202 b of the annulus202 rests on the outer chamber wall 170. The opening 204 faces thedisk-shaped dielectric window 112. The axial conductors 161-1 through161-8 (of the outer coil antenna 160) extend through respectiveinsulators 171 in the top gas plate 200.

The disk-shaped dielectric window 112 and the cylindrical dielectricside window 106 are heated and have their respective temperaturescontrolled independently of one another. They are heated and cooledindependently, by cooling from a fan system described later in thisspecification and by independent heater elements now described. A flatheater layer 220 depicted in FIGS. 1A, 7 and 8 overlies the disk-shapeddielectric window 112. The heater layer 220 is in the form of adisk-shaped Faraday shield, having an outer annulus 222 and pluralradial fingers 224 extending radially inwardly from the outer annulus222, the radial fingers 224 being separated from one another by evenlyspaced apertures 226. The spacing of the radial fingers 224 (definingthe width of the apertures 226) is sufficient to permit inductivecoupling of RF power through the heater layer 220. The heater layer 220is symmetrical with respect to the axis of the side wall 105. In theillustrated example, there are 24 radial fingers 224, although anysuitable number of fingers may be employed. The heater layer 220 isheated electrically by an internal electrically resistive element 229(FIG. 7) within the heater layer 220.

A cylindrical Faraday shield layer 230 depicted in FIGS. 1A and 9 isdisposed between the cylindrical dielectric window 106 and the outercoil antenna 160, and surrounds the cylindrical dielectric side window106. The cylindrical Faraday shield layer 230 has upper and lowercylindrical rings 232, 234, and plural axial legs 236 extending axiallybetween the upper and lower rings 232, 234 and being separated by evenlyspaced gaps 238. The cylindrical Faraday shield layer 230 may be heatedelectrically by an internal element (such as a heater layer 231 shown inFIGS. 1A and 7 within or contacting with the Faraday shield layer 230.

Process gas is injected into the processing region 101 by a centraldual-zone ceiling gas injector 300 (FIG. 1A) and a circular array ofperipheral (side) gas injectors 310 (FIG. 7). The ceiling gas injector300 is located at the center of the disk-shaped dielectric window 112.The peripheral gas injectors 310 are supported on the top gas plate 200near the side wall 106.

Referring to FIGS. 7, 10 and 11A, the lid assembly 110 includes anannular gas flow plate 320. The heater layer or Faraday shield 220 isheld on the gas flow plate 320 by a spring plate 322 as shown in FIG. 7.The gas flow plate 320 has three gas input ports 321 a, 321 b, 321 c(FIG. 10). The gas flow plate 320 provides recursive gas flow paths fromthe input port 321 a to a first zone of the dual zone ceiling gasinjector 300, recursive gas flow paths from the input port 321 b to theother zone of the dual zone gas injector 300, and recursive gas flowpaths from the gas input port 321 c to the side gas injectors 310. Theside gas injectors 310 are fed through respective gas ports 312 in thebottom surface of the gas flow plate 320 visible in the bottom view ofFIG. 11B. The recursive gas flow paths provide uniformly distributed gasflow path lengths to different gas injection zones. Uniformity controlof the gas distribution can also be enhanced by the recursive gas flowpaths.

Referring to FIG. 11A, a first set or hierarchy of recursive gas flowpaths 330 in the gas flow plate 320 feeds gas to the side gas injectors310 through the gas ports 312. The first set of recursive gas flow paths330 includes a half-circular gas flow path or channel 331. The gasinjection port 321 c is coupled to the midpoint of the half-circular gasflow channel 331. The gas flow path 331 extends around half a circle andfeeds at its ends the midpoints of a pair of arcuate gas flow paths 332each extending a quarter circle, which in turn feed at their respectiveends the midpoints of four arcuate gas flow paths 334 each extendingaround an eighth circle. The four arcuate gas flow paths 334 feed attheir ends the midpoints of eight arcuate gas flow paths 336 eachextending around a sixteenth of a circle. The ends of the gas flow paths336 feed the gas ports 312 for gas flow to the side gas injectors 310.

Referring to FIG. 12, gas flow to one zone of the dual zone gas injector300 is carried in a pair of opposing radial gas flow lines 340, 342overlying the disk-shaped dielectric window 112. Gas flow to the otherzone of the dual zone gas injector 300 is carried in a second pair ofopposing radial gas flow lines 344, 346 overlying the disk-shapeddielectric window 112 and disposed at right angles to the first pair ofradial gas flow lines 340, 342. Connection from the four radial gas flowlines 340, 342, 344, 346 to the dual zone gas injector 300 is providedby a gas flow hub 350 axially coupled to the dual zone gas injector 300.

Referring again to FIG. 11A, a half-circular gas flow channel 353provides uniform distribution of gas flow from the gas input port 321 bto the outer ends of the first pair of radial gas flow lines 340, 342. Aquarter-circular gas flow channel 357 provides gas flow from the inputport 321 b to the midpoint of the half-circular gas flow channel 353. Ahalf-circular gas flow channel 355 provides uniform gas flow from thegas input port 321 a to the outer ends of the second pair of radial gasflow lines 344, 346.

As depicted in FIGS. 12, 12A and 12B, each of the four radial gas flowlines 340, 342, 344, 346 overlying the disk-shaped dielectric window 112may be enclosed in a respective one of the radial fingers 224 of theheater layer 220.

As previously described with reference to FIGS. 1 and 12, the gas flowhub 350 provides coupling between the four radial gas flow lines 340,342, 344, 346 and the dual zone gas injector 300. One example of thedual zone gas injector 300 is depicted in FIG. 13. The dual zone gasinjector 300 of FIG. 13 includes a center gas disperser 302 having anaxial inner annular channel 302-1 extending axially and dispersing gasto a radially inner zone A, and a middle gas disperser 304 having aslanted middle annular channel 304-1 dispersing gas to a radially outerzone B. The gas flow hub 350 is now described with reference to FIGS.13, 14A, 14B, 15 and 16. The hub 350 has four gas inlet ports 352, 354,356 and 358 oriented at right angles to one another and connectable tothe four radial gas flow lines 340, 342, 344, 346 as indicated in dashedline. The gas inlet ports 352 and 354 feed respective pairs of split gasdistribution lines 360, 362, respectively, that terminate at fourequally spaced points along a circular inner distribution channel 366that is in registration with the axial inner annular channel 302-1 ofthe dual zone gas injector 300. The gas inlet ports 356 and 358 feedrespective pairs of split gas distribution lines 370, 372, respectively,that terminate at four equally spaced points along a circular middledistribution channel 374 that is in registration with the axial middleannular channel 304-1 of the dual zone gas injector 300.

Referring again to the bottom view of FIG. 11B, in one embodiment,optional cooling passages 390 may be provided in the gas flow plate 320,in the form of a circular supply passage 390 a and a circular returnpassage 390 b forming a continuous path. External coolant ports 392 aand 392 b provided connection of the supply and return passages 390 a,390 b to an external coolant source (not illustrated in FIG. 11B).Optionally, internal coolant passages may be provided in the outerchamber body wall 170 and fed through a coolant input port.

Referring to FIGS. 1 and 1B, the chamber liner 107 is enclosed within alower chamber body 400 including a cylindrical lower chamber body sidewall 405 and a lower chamber body floor 410. The lower chamber body sidewall 405 and the lower chamber body floor 410 enclose an evacuationregion 411. The chamber liner 107 includes an upper cylindrical section107-1 and a lower annular grid 107-2 in the form of an invertedtruncated cone. A vacuum pump 440 is disposed in a vacuum pump opening410 a in the floor 410 and is centered relative to the axis of symmetryof the side wall 105. A containment wall 415 coaxial with the workpiecesupport 115 and a flexible bellows 417 extending between the pedestal120 and the containment wall 415 enclose the workpiece support 115 in aninternal central space 419. The central space 419 is isolated from thevolume evacuated by the vacuum pump 440, including the evacuation region411 and the processing region 101. Referring to FIGS. 1B, 17 and 18,there are three hollow radial struts 420 defining radial access passages421 spaced at 120 degree intervals extending through the chamber bodyside wall 405 and providing access to the central space 419. Three axialexhaust passages 430 are defined between the three radial struts 420.Different utilities may be provided through different ones of the radialaccess passages 421, including the RF power cable 132 connected to theelectrode 130, heater voltage supply lines connected to heater elementsin the workpiece support 115, an electrostatic chucking voltage supplyline connected to the electrode 130, coolant supply lines and heliumsupply lines for backside helium gas channels in the workpiece supportsurface 121, for example. A workpiece support lift actuator 450 is fixedwith respect to the chamber body and moves the workpiece support 115axially. The workpiece support lift actuator 450 may be used to vary thedistance between the workpiece 122 and the lid assembly 110. Varyingthis distance varies the distribution of plasma ion density. Movement ofthe lift actuator may be used to improve uniformity of distribution ofprocess (e.g., etch) rate across the surface of the workpiece 122. Thelift actuator 450 may be controlled by the user through a programmablecontroller, for example.

The axially centered exhaust assembly including the vacuum pump opening410 a and the axial exhaust passages 430 avoids asymmetries or skew inprocessing distribution across the workpiece 122. The annular grid 107-2masks the processing region 101 from the discontinuities or effects ofthe radial struts 420. The combination of the axially centered exhaustassembly with the symmetrical distribution of RF current flow below theground plate 184 minimize skew effects throughout the processing region101, and enhance process uniformity in the processing region 101.

FIG. 19 depicts cooling air flow through the upper section 20 of FIG.1A. Referring to FIGS. 1A and 19, a chamber body side wall 406 surroundsthe lid assembly 110. A lower plenum wall 500 in the form of a truncatedcone, for example, is mounted between the top edge of the chamber bodyside wall 406 and the peripheral edge of the ground plate 184, toenclose a lower plenum 502. A circular array of exhaust fans 504 aremounted in respective openings 506 in the lower plenum wall 500.

The ground plate 184 has a center opening 600 that is co-extensive withthe inner ground shield 149. A cylindrical plenum center wall 606 iscoextensive with the center opening 600. A plenum plate 610 overlies theplenum center wall 606. A return chamber 612 is enclosed between areturn chamber side wall 608, the plenum plate 610, the ground plate 184and the center wall 606. The return chamber side wall 608 includes airflow screen sections 609. Openings 614 through the ground plate 184permit air flow between the lower plenum 502 and the return chamber 612.

An upper plenum 650 is enclosed between a top plate 655 and the plenumplate 610 by an upper plenum side wall 660 in the form of a truncatedcone. Plural intake fans 665 are mounted at respective openings 667 inthe upper plenum side wall 660.

The intake fans 665 draw air into the upper plenum 650 which flows downthrough the central opening formed by the center wall 606, the groundplate opening 600 and the middle grounded shield 149. An annular airflow plate 670 overlying the disk-shaped dielectric window 112 confinesthe air flow between the plate 670 and the window 112. This air may flowthrough the apertures 226 of the Faraday shield 220 of FIG. 8, forexample. Alternatively (or in addition), the air may be confined in agap between the air flow plate 670 and the window 112. Downward air flowthrough the cylindrical shield 149 enters the space within each aperture226 through a central opening of the plate 670 and flows radiallyoutwardly over the disk-shaped dielectric window 112 and enters thelower plenum 502. From the lower plenum 502, the air escapes into thereturn chamber 612, from which it may exit through the screen sections609 of the return chamber side wall 608. Thus, the intake fans 665provide cooling for the disk-shaped dielectric window 112.

The exhaust fans 504 provide cooling for the cylindrical dielectricwindow 106. The exhaust fans 504 draw air through intake ports 680 inthe lower chamber side wall 170 and past the cylindrical dielectricwindow 106. By operating the intake fans 665 independently from theexhaust fans 504, the different heat loads on the different dielectricwindows 106 and 112 may be compensated independently, for accuratetemperature control of each window.

FIG. 20A depicts one embodiment of an RF source for the three coilantennas 140, 150, 160, the RF source having independent RF generators740-1, 740-2, 740-3, and RF impedance matching networks 742-1, 742-1,742-3 for the respective coil antennas 140, 150, 160. FIG. 20B depictsan embodiment in which the inner and middle coil antennas 140, 150 aredriven from a single RF generator 750-1 through an RF impedance matchingnetwork 180 having dual outputs. The dual output RF impedance matchingnetwork 180 may facilitate differential control of the power levelsapplied to the inner and middle coil antennas 140, 150. The outer coilantenna 160 is driven by an RF generator 750-2 through an RF impedancematching network 182. The dual output RF impedance matching network 180functions as two separate RF power sources, so that there are a total ofthree RF power sources in the system. In each of the foregoingembodiments, the RF impedance matching networks may be disposed on thetop plate 655 as depicted in FIG. 1A.

FIG. 21 depicts a control system for controlling the plasma reactor ofFIG. 1. The control system is responsive to temperature sensors atdifferent locations within the plasma reactor, such as a temperaturesensor 106′ at or in the cylindrical dielectric window 106, and atemperature sensor 112′ at or in the disk-shaped dielectric window 112.The control system includes a programmable controller 800 which may beimplemented as a microprocessor, for example. The controller 800 has aninput 802 for receiving the output of the temperature sensor 106′ and aninput 804 for receiving the output of the temperature sensor 112′. Thecontroller 800 has independent command outputs, including an output 806governing the speed of the intake fans 665, an output 808 governing thespeed of the exhaust fans 504, an output 810 governing the flow rate ofcoolant to the coolant port 392 a in the gas flow plate 320, an output812 governing the power level to the electric heater 229 near thedielectric window 112, and an output 814 governing the power level tothe electric heater 231 at the cylindrical dielectric window 106.

The controller 800 in one embodiment is programmed to govern the outputs808-814 in response to the inputs 802, 804 to maintain the windows 106,112 at respective target temperatures that may be furnished by a user tocontroller inputs 816 and 818. The controller 800 may be programmed tooperate in the manner of a feedback control loop to minimize thedifference between the user input 816 and the sensor input 802, and tominimize the difference between the user input 818 and the sensor input804.

As described above, some of the advantageous effects of various ones ofthe foregoing embodiments include symmetrical distribution of RF powerto the coil antennas for enhanced plasma distribution symmetry.Shielding of the coils from asymmetrical RF current feed structuresreduces skew effects in plasma distribution. Shielding of the coilantenna feeds from one another enhances independent control of the coilantennas, for superior control of plasma density distribution.Symmetrical chamber exhaust in combination with the symmetrical coilantennas provides a high density plasma source with symmetrical plasmadistribution. Separate dielectric windows for different RF coils enablesindependent thermal control of the different dielectric windows.Separately supporting the different dielectric windows at or over theprocessing region enables the chamber diameter to be increased beyondthe diameter of each individual dielectric window, facilitating a largeincrease in chamber diameter. The moveable workpiece support electrodein combination with symmetrical coil antenna(s) allows superior controlover center-to-edge plasma density distribution with a minimizedasymmetrical non-uniformity component. The moveable workpiece supportelectrode in combination with symmetrical coil antenna(s) and in furthercombination with the symmetrical chamber exhaust allows even bettercontrol over center-to-edge plasma density distribution with minimizedasymmetrical non-uniform component.

FIGS. 22-27 depicts an RF feed structure which can have triangularsymmetry, in one embodiment. The embodiment of FIGS. 22-27 is amodification of the embodiment of FIG. 1A and includes elementsdescribed above with reference to FIG. 1A, for which identical referencenumerals are used. The embodiment of FIGS. 22-28 includes a ceilingplate 900 overlying a cylindrical side wall 902 above the top plate 655.Three RF power terminals 904 a, 904 b 904 c are provided at the ceilingplate 900, and may be housed in a connection module 904 held in theceiling plate 900. A center RF connection rod 906 is connected betweenthe RF power terminal 904 a and the RF current distributor 142 for theinner coil 140.

RF power for the middle coil 150 is distributed to the four axial RFfeed rods 158 by a middle RF distribution ring 908 having four radialarms 910 a-910 d (FIG. 26) connected to the tops of the respective RFfeed rods 158. The middle RF distribution ring 908 is disposed above andspaced from the conductive ground plate 184. A middle RF connection rod912 is connected between the RF power terminal 904 b and the middle RFdistribution ring 908.

The ground plate 184 has a center opening 600. In one embodiment, thecenter opening 600 may be covered by a conductive grill 970 that permitsair flow through the center opening 600.

RF power for the outer coil 160 is distributed to the eight axial RFfeed rods 168 by an outer distribution cylinder 914. In one embodiment,the outer distribution cylinder 914 is coaxial with the axis of symmetryof the coils 140, 150 and 160. The outer distribution cylinder 914 has atop portion 914 a (FIG. 25) above the top plate 655 and a bottom portion914 b below the plenum plate 610. Eight radial arms 915 a-915 h (FIG.25) extend from the bottom 914 b of the outer distribution cylinder 914to respective ones of the eight axial RF feed rods 168. An outer RFconnection rod 916 (FIGS. 24 and 25) is connected between the top 914 aof the outer distribution cylinder 914 and the RF power terminal 904 c.The outer distribution cylinder 914 is hollow in one embodiment andsurrounds the center RF connection rod 906 and the middle RF connectionrod 912. The outer distribution cylinder 914 extends through openings610 a, 655 a in the grounded plates 610 and 655 respectively. Thediameters of the openings 610 a and 655 a are sufficient to provide gaps611, 656, respectively, between the grounded plates 610, 655 and theouter distribution cylinder 914. In one embodiment, the gaps 611 and 656are sufficiently wide to prevent electrical discharge or breakdown atoperating (e.g., atmospheric) pressure, given the RF voltage supplied atthe RF power terminal 904 c.

The bottom 914 b of the outer distribution cylinder 914 is at an axiallocation above the middle RF distribution ring 908. In one embodiment,the outer distribution cylinder 914 and the middle RF distribution ring908 are coaxial and axially separated from one another, and may be ofthe same diameter. In one embodiment, the middle RF distribution ring908 fits between the bottom 914 b of the outer distribution cylinder 914and the grounded plate 184. The middle RF distribution ring 908 isseparated from the bottom 914 b of the outer distribution cylinder 914and from the ground plate 184. In one embodiment, the axial length ofthe middle RF distribution ring 908 is less than the axial distancebetween the bottom 914 b of the outer cylinder 914 and the ground plate184.

In one embodiment, the edge of the top portion 914 a of the outerdistribution cylinder 914 lies adjacent a radial plane coinciding withsection line 24A-24A of FIG. 24. The three RF connection rods 906, 912and 916 extend axially and intersect the radial plane of the top edge ofthe outer distribution cylinder 914 at three points 907, 913 and 917(FIG. 24A) respectively. In one embodiment, the three points ofintersection 907, 913 and 917 are symmetrically arranged as the verticesof an equilateral triangle, providing a triangular symmetry within theouter distribution cylinder 914. In one embodiment, the RF connectionrods 906, 912 and 916 have radial transition sections 906 a, 912 a and916 a, respectively, that locate the top ends of the rods 906, 912 and916 in registration with respective ones of the RF power terminals 904a, 904 b and 904 c. The radial transition sections 906 a, 912 a and 916a are above the radial plane coinciding with the section line 24A-24A.

FIGS. 28-31 depict a modification of the embodiment of FIGS. 22-27, inwhich, in one embodiment, the RF feed structure has coaxial symmetry. Acenter RF connection rod 920 is connected between the RF power terminal904 a and the RF current distributor 142 for the inner coil 140. In oneembodiment, the center RF connection rod 920 is straight and coincideswith the axis of symmetry of the coil 140.

In the embodiment of FIGS. 28-31, RF power for the middle coil 150 isdistributed to the four axial RF feed rods 158 by a middle RFdistribution tube 922. The middle RF distribution tube 922 is hollow andcylindrical in one embodiment. The middle RF distribution tube 922 has atop portion 922 a (FIG. 31) above the top plate 655 and a bottom portion922 b below the plenum plate 610. Four radial arms 924 a-924 d extendradially outwardly from the bottom portion 922 b of the middle RFdistribution tube 922 to the tops of the respective RF feed rods 158. Inone embodiment, the top portion 922 a of the middle RF distribution tube922 has an annular flange 922 c extending radially outwardly from the RFdistribution tube 922. An axial middle RF connection rod 925 mounted onthe annular flange 922 c extends upwardly to the RF power terminal 904b. In one embodiment, the middle RF distribution tube 922 surrounds andis coaxial with the center RF connection rod 920.

In the embodiment of FIGS. 28-31, RF power for the outer coil 160 isdistributed to the eight axial RF feed rods 168 by an outer distributioncylinder 930. In one embodiment, the outer distribution cylinder 930 ishollow and is coaxial with the axis of symmetry of the coils 140, 150and 160.

The outer distribution cylinder 930 has a top portion 930 a (FIG. 30)above the top plate 655 and a bottom portion 930 b below the plenumplate 610. Eight radial arms 931 a-931 h extend from the bottom portion930 b of the outer distribution cylinder 930 to respective ones of theeight axial RF feed rods 168. In one embodiment, the top portion 930 aof the outer distribution cylinder 930 has an annular flange 930 c lyingin a radial plane and extending radially outwardly from the outerdistribution cylinder 930. In one embodiment, the annular flange 930 cis axially below the level of the annular flange 922 c of the middle RFdistribution tube 922. An outer connection rod 932 extends upwardly fromthe annular flange 930 c to the RF power terminal 904 c. The outerdistribution cylinder 930 surrounds and is coaxial with the middle RFdistribution tube 922. The outer distribution cylinder 930 extendsthrough openings 610 a, 655 a in the grounded plates 610 and 655respectively. The diameters of the openings 610 a and 655 a aresufficient to provide respective gaps 611, 656 between the respectiveplates 610, 655 and the outer distribution cylinder 930. In oneembodiment, the gaps 611 and 656 are sufficiently wide to preventelectrical discharge or breakdown at operating (e.g., atmospheric)pressure, given the RF voltage supplied at the RF power terminal 904 c.

FIGS. 32-34 depict a modification of the embodiment of FIGS. 28-31 inwhich spaced-apart capacitors 934 are connected between the outerdistribution cylinder 930 and the ceiling plate 655. The capacitors 934are connected in parallel and uniformly spaced in one embodiment andtheir capacitance is selected to provide resonance at the frequency ofthe RF power applied to the outer coil 160. The effect of such resonanceis to enhance uniformity of RF power distribution in the RF feedstructure including the outer distribution cylinder 930. In oneembodiment, an axial lip 930 d extends upwardly from the outer edge ofthe axial flange 930 c, and is polygonal in shape, having plural flatfaces on which respective ones of the capacitors 934 are mounted andconnected to the axial lip 930 d. A right angle bracket 936 is providedfor connecting the outwardly facing end of each capacitor 934 to theceiling plate 655. In the illustrated embodiment, there are eightcapacitors 934 and the axial lip 930 d has eight faces, although anysuitable number of capacitors and faces may be employed.

The array of capacitors 934 of FIGS. 32-34 may also be incorporated intothe embodiment of FIG. 22. In one implementation, the capacitors areconnected between the outer distribution cylinder 914 of FIG. 22 and theceiling plate 655, for example.

FIGS. 35-36 depict a modification of the embodiment of FIGS. 32-34,having electromagnetic shielding between the RF feeds to the three coils140, 150, 160. In one embodiment, this feature enhances theirindependent control. In FIGS. 35-36, the outer distribution cylinder 930is replaced by a shortened outer distribution cylinder 940 that extendsdownwardly from its top end 940 a by a short distance to its bottom end940 b and does not reach the plenum plate 610. A radial flange 940 cextends outwardly from the top end 940 a of the shortened distributioncylinder 940. A skirt 942 extends radially outwardly from the bottom end940 b of the shortened distribution cylinder 940. RF feed rods 944extend axially through individual holes in the plenum plate 610 andthrough openings in the ground plate 184 without electrically contactingthe plenum plate 610 or ground plate 184. The RF feed rods 944 areconnected at their top ends to the periphery of the skirt 942 and areconnected at their bottom ends to the top of the RF current distributor162 for the outer coil 160.

An inner shield 950 surrounds the center RF connection rod 920 and lieswithin the hollow interior of the middle RF distribution tube 922. Inone embodiment, the inner shield 950 is a hollow cylinder having aradially extending flange 950-1 at its top end. The inner shield 950extends down to (and contacts) the conductive grill 970, where it isterminated. In one embodiment, the inner shield 950 electromagneticallyshields the RF connection rod 920 and the middle RF distribution tube922 from each other. In one embodiment, the inner shield 950, the RFconnection rod 920 and the middle RF distribution tube 922 are coaxial.The inner shield 950 is separated from the RF connection rod 920 andfrom the middle RF distribution tube 922 by cylindrical gaps 921 and923, respectively, that are sufficient to prevent discharge or breakdownin the gaps 921 and 923.

An upper shield 955 is disposed between the middle RF distribution tube922 and the shortened RF distribution cylinder 940 including a radialflange 955-1 at its top end. In one embodiment, the upper shield 955 isa hollow cylinder coaxial with the shortened RF distribution cylinder940 and coaxial with the middle RF distribution tube 922, and surroundsthe middle RF distribution tube 922. The upper shield 955 extends downto an opening 610 a in the plenum plate 610. A skirt 957 extendsradially out from a bottom edge 955 a of the upper shield 955 and has asufficient radius to overlie an edge of the opening 610 a in the plenumplate 610, so as to cover a separation gap between the plenum plate 610and the upper shield 955.

A middle cylindrical shield 960 extends axially from the plenum plate610 and towards the ground plate 184. In an embodiment, the middlecylindrical shield 960 is coaxial with the shields 950 and 955. A lowercylindrical shield 965 extends downwardly from the ground plate 184 to abottom edge 965 a. A bottom shield 971 including a hollow truncatedcone-shaped section, extends downwardly from the bottom edge 965 a ofthe lower cylindrical shield 965 to a cylindrical base 972. Thecylindrical base 972 encircles a floor 974 of the bottom shield 971.

Advantageous effects of various ones of the embodiments of FIGS. 22-36include symmetrical distribution of RF power to the coil antennas forenhanced plasma distribution symmetry. For example, the RF feedstructure of FIG. 22 has triangular symmetry, as defined with referenceto FIGS. 22 and 24A. Shielding of the coils from asymmetrical featuresreduces skew effects in plasma distribution. Such shielding is providedin the embodiments of FIGS. 22-36 by the ground plate 184, the plenumplate 610 and the ceiling plate 655. Such shielding is further providedby the flanges 922 c and 930 c and the skirts 942 and 957, for example.The extra height afforded by the addition of the ceiling plate 900 abovethe top plate 655 and raising the RF power terminals to the ceilingplate 900 improves RF current distribution uniformity. It is believedthat this added height increases the length of RF current travel throughthe RF feeds over which current distribution may equilibrate to increaseuniformity. Shielding of the coil antenna feeds from one anotherenhances independent control of the coil antennas, for superior controlof plasma density distribution.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. A plasma reactor comprising: a window assembly;inner, middle and outer coil antennas adjacent said window assembly, andinner, middle and outer current distributors coupled to said inner,middle and outer coil antennas, respectively; a ceiling plate overlyingsaid window assembly and first, second and third RF power terminals atsaid ceiling plate; first, second and third axial RF power feedsconnected between respective ones of said first, second and third RFpower terminals and respective ones of said inner, middle and outercurrent distributors; wherein said third axial RF power feed comprises ahollow axial outer RF power distribution cylinder surrounding said firstand second axial RF power feeds.
 2. The plasma reactor of claim 1wherein said second axial RF power feed comprises a hollow axial middleRF power distribution cylinder surrounding said first axial RF powerfeed.
 3. The plasma reactor of claim 2 wherein said first axial RF powerfeed comprises a center RF connection rod, and wherein said center RFconnection rod, said hollow middle RF power distribution cylinder andsaid outer RF distribution cylinder are coaxial.
 4. The plasma reactorof claim 3 wherein said inner, middle and outer coil antennas arecoaxial with said center RF connection rod.
 5. The plasma reactor ofclaim 3 further comprising: a plenum plate spaced from and below saidceiling plate and comprising a central opening.
 6. The plasma reactor ofclaim 5 wherein said outer RF distribution cylinder extends through saidcentral opening with a gap between said plenum plate and said outer RFdistribution cylinder, said plasma reactor further comprising: a radialflange extending from said outer RF distribution cylinder and overlyingsaid gap.
 7. The plasma reactor of claim 2 further comprising: pluralradial middle arms extending outwardly from said middle RF distributioncylinder and plural axial middle legs extending from said plural radialmiddle arms to spaced apart locations on said middle currentdistributor; plural radial outer arms extending outwardly from saidouter RF distribution cylinder and plural axial outer legs extendingfrom said plural radial arms to spaced apart locations on said outercurrent distributor.
 8. The plasma reactor of claim 7 further comprisinga ground plate below said ceiling plate and above said middle currentdistributor, said plural axial middle legs extending through said groundplate.
 9. The plasma reactor of claim 8 wherein said plural axial outerlegs extend through said ground plate.
 10. The plasma reactor of claim 8wherein: each one of said inner, middle and outer coil antennascomprises plural conductors having RF supply ends; each one of saidinner, middle and outer current distributors comprises an axiallysymmetric hollow body comprising a bottom facing the supply ends of theplural conductors of the corresponding one of said inner, middle andouter coil antennas.
 11. The plasma reactor of claim 5 furthercomprising: plural spaced-apart reactance elements coupled to said outerRF power distribution cylinder.
 12. The plasma reactor of claim 11wherein said reactance elements comprise discrete capacitors coupledbetween said RF power distribution cylinder and said ceiling plate. 13.The plasma reactor of claim 8 further comprising: a hollow cylindricalinner shield between said center RF connection rod and said middle RFdistribution cylinder; and a hollow cylindrical upper shield between themiddle RF distribution cylinder and the outer RF distribution cylinder,said upper shield comprising a bottom edge.
 14. The plasma reactor ofclaim 13 further comprising a plenum plate spaced from and below saidceiling plate and comprising a central opening, said upper shieldextending through said central opening and defining an inner gap betweensaid plenum plate and said upper shield.
 15. The plasma reactor of claim14 further comprising: a skirt extending radially out from said uppershield near said bottom edge and overlying said inner gap.
 16. Theplasma reactor of claim 13 further comprising: a middle cylindricalshield extending axially toward said ground plate and surrounding saidplural axial middle legs; a lower cylindrical shield extendingdownwardly from said ground plate; a bottom shield extending downwardlyfrom the bottom edge of the lower cylindrical shield.
 17. The plasmareactor of claim 16 further comprising: a cylindrical base extendingaxially from said bottom shield, said cylindrical base surrounding saidmiddle coil.
 18. The plasma reactor of claim 17 wherein said bottomshield comprises a hollow truncated cone.
 19. The plasma reactor ofclaim 17 further comprising a floor encircled by said cylindrical base.20. The plasma reactor of claim 1 wherein said outer coil antennacomprises a cylindrical side coil and wherein said window assemblycomprises a disk-shaped window facing at least one of said inner andmiddle coil antennas and a cylindrical side window facing saidcylindrical side coil.